1. Field of the Invention
The present invention relates to an image developing apparatus to be incorporated, for example, in a copy machine, for developing a latent image by means of a toner and, more particularly, to such an image developing apparatus capable of developing high quality images without a fog and a toner density fluctuation over an extended period of time while reducing a driving torque in an image developing operation.
2. Description of the Prior Art
A conventional image forming apparatus such as a copy machine is normally equipped with an image developing apparatus which utilizes a toner of binary system or of magnetic single component system as a developer, in order to develop a latent image formed on a photosensitive body.
However, such an image developing apparatus has a drawback of being difficult to convert to a color image developing apparatus due to the fact that a fine control of toner-carrier ratio is necessary for the binary system toner whereas the magnetic single component toner contains magnetic bodies of dark complexion. Moreover, it is necessary to equip such a conventional image developing apparatus with an expensive magnetic roller as a developing roller which functions as a toner conveying means.
To cope with this situation, there is a recent proposition to use a non-magnetic single component toner. An image developing apparatus using such a non-magnetic single component toner incorporates a hopper equipped with a mixer and a toner supply roller which supplies a toner to a developing roller by means of their rotational motion, and an elastic blade which functions as a lamination controlling means, to be placed around the developing roller in order to form a toner lamina of approximately 30 .mu.m thickness on the developing roller. The latent image on a photosensitive body which is an image bearer is developed by placing this toner lamina in a proximity of the photosensitive body.
In such an image developing apparatus using a non-magnetic single component toner, the toner is of positively electrifiable type which has an electrification of approximately +12 .mu.C/g after being laminated, as oppose to the negatively electrifiable photosensitive body. This manner of developing the latent images is in principle that using a highly resistive single component toner, so that the electrification of the toner is accomplished by means of the electrification due to a friction between the developing roller and the elastic blade. In this case, a magnetic roller is not necessary because the conveying of the toner is accomplished by means of the rotational motion of the developing roller. Thus, this image developing apparatus using a non-magnetic single component toner has been considered to be capable of developing high quality images by a simple configuration, inexpensively, and is suitable for converting into a color image developing apparatus.
However, it has been realized that this image developing apparatus using a non-magnetic single component toner is also associated with problems. In a first place, abrasion of the developing roller and the elastic blade causes an considerable amount of the reduction of the image density as well as the blurring of the letters after an extensive use. Secondly, a large friction between the toner and the elastic blade made of a metal or a rubber causes an increase of a driving torque in an image developing operation. Thirdly, the quality of the image is uneven depending on the types of the toner, such that a fog tends to be produced when the poorly electrifiable toner is used, while the image density tends to lower when a highly electrifiable toner is used.